In a permanent magnet synchronous motor (hereinafter simply referred to as “motor”) driven by a current amplifier (hereinafter simply referred to as “amp”), the maximum values for current and voltage are limited by the constraints of the amp. In order to obtain maximum output under current and voltage limiting, it is preferable to perform optimum current vector control.
In a current vector control wherein a motor is controlled on the basis of an orthogonal dq coordinate system, when the motor is driven in a high-speed range in which a motor electromotive force surpasses the maximum voltage of the amp, a reactive d-phase current is applied in order to bring a motor terminal voltage to be less than the maximum voltage of the amp. By performing field weakening by applying the d-phase current in a current control (including current command generation), an increase of the motor terminal voltage due to a speed increase is inhibited, and the stable output operation range can be expanded. However, when the d-phase current is not adequately controlled, an increase of the d-phase current may lead to heat generation of the motor, and a shortage of the d-phase current may lead to voltage saturation. In other words, a voltage across motor terminals is limited by the maximum voltage which the amp can output, whereby a current control may become unstable.
In order to prevent this situation, methods have been proposed wherein the d-phase current is adequately applied corresponding to a motor load (in reality a torque command) (see, e.g., Japanese Unexamined Patent Publication No. JP-A-2003-052199 (hereinafter called “Patent Literature 1”)). Specifically, the optimum d-phase current at no load, and the optimum d-phase current at maximum load are calculated and used to calculate q-phase and d-phase current commands corresponding to the load (see Formulas (16) and (17) of Patent Literature 1). In this case, the difference between no load and maximum load is determined by a primary linear approximation, so that an excessive d-phase current might flow. Therefore, in this conventional art, a voltage command, which is the output of the current control, is further used to adjust a speed (base speed) that is the reference for starting the application of the d-phase current, thereby improving the excessive state of d-phase current, and inhibiting heat generation of the motor (see FIG. 3 of Patent Literature 1).
However, in this conventional art, a problem occurs in motors wherein the short-circuit current determined by the ratio of the magnet's main magnetic flux and inductance is smaller than the maximum current of the amp; the problem is that the d-phase current for performing maximum torque/magnetic flux control may not be correctly applied, when maximum output control is performed by considering solely the voltage limiting during a high-speed operation of the motor. In this case, “maximum torque/magnetic flux control” is a method in a synchronous motor equipped with magnets wherein the phase difference between the q-phase current and the d-phase current is controlled such that the total magnetic flux, which corresponds to the motor terminal voltage upon application of an armature current corresponding to the preferable torque, is minimized.
On the other hand, methods have been proposed to prevent destabilization of current control due to voltage saturation wherein the q-phase current command is corrected on the basis of the result of proportion and integral of the error between the voltage command and a reference value (see, e.g., Japanese Unexamined Patent Publication No. JP-A-2000-341990). However, while stability during voltage saturation is improved in this conventional art, no consideration is given to the application of an optimum d-phase current and q-phase current corresponding to the motor speed, and therefore, an optimum current control is not performed.
Further, methods have been proposed wherein the d-phase current command is determined by a simple formula using the torque command, and is used to correct the q-phase current command (see, e.g., Japanese Patent Publication No. JP-4045307). An optimum current can be hereby applied, and a maximum torque control can be realized. However, no consideration is given to voltage saturation in this conventional art, so that such a problem occurs when an optimum current control may not be performed in the high-speed range. Further, since the exact values for the motor constants are preferable in the simple formula, there are problems of stability when constants fluctuate.
The present invention proposes a current control device of a synchronous motor wherein a voltage command being the output of the current control is used to correct the d-phase current command in order to adequately perform field weakening in the high-speed range (voltage saturation range) in the current control.